How it works

UVA Context is a model, not a measurement.

Beyond UV shows two numbers side by side: the official UV Index from Apple Weather, and UVA Context, an estimate of longer-wave UVA (315–400 nm) that Beyond UV calculates on your device. The two are never added or averaged.

The UV Index is only part of the story. UV Index: the familiar UV signal, Standard UV Index. UVA Context: a new UVA perspective, Beyond UV's modeled estimate.

Why a model?

Weather services forecast the UV Index, which is weighted toward the UVB that causes sunburn. There is no public hourly UVA forecast for consumer apps, so Beyond UV estimates UVA from the inputs that drive it.

What goes into it

The sun's position for your place, date, and time, computed on your device.
The season and your elevation. UVA rises with altitude.
How much sunlight the atmosphere is letting through today. Beyond UV compares Apple Weather's hourly UV Index forecast with the clear-sky value it expects for that hour, and adds cloud amount, cloud height, and whether the sun is likely behind cloud.

The result is placed on the same 0–11+ scale and Low-to-Extreme categories as the UV Index, so the two are easy to compare. The UV Index is never used as a stand-in for UVA; it only informs how much light is getting through.

What it is not

Not the official UV Index, and not a measurement of UVA.
Not your personal UV dose. Shade, glass, clothing, sunscreen, and reflective surfaces all change what reaches your skin.
Not a count of minutes that are "safe".

How accurate is it?

The model's coefficients were fitted against NASA POWER, a satellite and reanalysis dataset that is itself a model, not ground measurement. It has not yet been validated against ground-based UVA instruments, so no accuracy figure is published. It is least reliable in the first and last hours of daylight, and the cloud-height and haze parts are the least tested. When the Apple Weather forecast is unavailable, the app falls back to a clear-sky estimate and labels it.

How Beyond UV uses it

Today's headline level is the higher category of the UV Index and UVA Context. That's why the page can stay elevated in the late afternoon, when the UV Index has eased but UVA, which changes less through the day, is still present.

In the app, tap the UVA Context tile on Today, then "How UVA Context is calculated", for the full method.